1. Field of the Invention
The present invention relates to a transmission control unit for a vehicular automatic transmission. More particularly, the invention is directed to a transmission control unit designed to effect shifts suitable for heavy traffic.
2. Description of the Related Art
In order to prevent frequent shifting between low speed gears in an automatic transmission, in response to a slight change in speed when a vehicle runs through heavy traffic, and the uncomfortable ride resulting from same, attempts have been made to effect shifts suitable for heavy traffic (see Japanese laid-open patent publication No. 4-50549). Specifically, when vehicle speed and throttle position continue to be less than a predetermined value for a given period of time, the traffic is considered to be heavy. Within the transmission control unit, a predetermined normal traffic transmission map is changed to a heavy traffic transmission map. At a given throttle position a second gear is maintained longer when operating under control of the heavy traffic transmission map than when operating with the normal traffic transmission map.
The prior art attempts are effective when the traffic is truly or continuously heavy, but may not function efficiently when vehicle speed temporarily increases while the traffic is heavy. If the vehicle speed increases a given amount, a timer within the transmission control unit is reset, the traffic is then considered no longer to be heavy and, as a result, shifts are made in accordance with the normal traffic transmission map.
In order to eliminate the above-described problem, traffic conditions could be determined by calculating average vehicle speed for a fixed period of time rather than by simply measuring vehicle speed at each interval of time. However, stand-by time is required to make such a calculation, resulting in a delayed determination. To minimize such delay, measurement time could be shortened but the result would be an accuracy no better than that obtained when the vehicle speed is simply measured at regular intervals. Thus, this latter approach is not effective to solve the above-described problem.